THE NUTRITION OF THE ROOT 



31 



{a.) the absolutely necessary or essential nutritive substances, 

 which must always be present in sufficient quantity and in 

 soluble form : these are oxygen, hydrogen, nitrogen, carbon, 

 sulphur, phosphorus, potassium, calcium, magnesium, iron, 

 and probably also chlorine ; (b.) subsidiary substances, which 

 are present in every ash, but are not indispensable for the 

 growth of plants : sodium, silica ; (c.) admixtures which are 

 due to the nature of the soil : such as zinc, copper, nickel, 

 aluminium, cobalt, manganese, lithium, strontium, and barium. 



§ 5. What is the eflFect of the various nutritive substances on 

 the plant, and in what form do they enter it ? 



All tissues of the plant contain oxyg-en, hydrog-en, and 

 carbon. The first two substances are not only necessary to 

 the plant in the construction of its tissues, but they are 

 essential for purposes of conduction ; for they make up the 

 water which pervades the whole plant. Tlie percentage of 

 water present in the various parts of the plant is naturally 

 very variable. Succulent herbaceous tissues possess 60 to 



7 5 per cent, of water, while the fungi often possess only 5 to 



8 per cent, of organic substance, the rest of their weight being 

 made up of water. As most of the nutritive salts are con- 

 tained in the soil, we can easily form an opinion of the 

 importance of water, which is the medium by which the roots 

 have to absorb the salts. These solutions pass through the 

 vessels of the plant, and the adjoining cells can absorb water 

 from the vessels, though they have often to pass it on to 

 neighbouring cells, and thus a constant absorption of the 

 water contained in the soil is brought about. Nearly all the 

 water which passes through the plant, carrying with it the 

 nutritive salts, is taken up by the root. This water is ulti- 

 mately given off as water-vapour, or is chemically decomposed 

 and used up in the construction of new material. Only in 

 cases of extreme dryness of the soil are the leaves able to 

 make use of the heavy deposits of dew. 



In the case of carbon, however, which must be looked upon 

 as the chief constituent of the vegetable tissues, forming as it 

 does one half of their total dry weight, the soil is of no use to 



